1. Field of the Invention
The present invention relates to the improvement of a flicker compensating apparatus for a DC arc furnace, which reduces the amount of flickering caused by a variation in load of the DC arc furnace which melts a . raw material to be molten (hereinafter called "melting stock"), such as iron scrap, direct reduced iron, and pig iron, using DC arc discharge.
2. Description of the Related Art
In general, when the desired current is applied to a graphite electrode inserted in an arc furnace, arc discharge occurs between this electrode and melting stocks placed in the furnace and melts the melting stocks. The arc discharge can easily create a high-temperature environment, has a high energy density, and has an advantage of easiness in controlling the arc current. In this respect, the arc discharge is widely utilized in melting of melting stocks.
Conventionally, a three-phase AC arc furnace has been used for steel making to melt melting stocks. However, increased consumption of steel demands a greater furnace of which productivity increases in the amount of melting stocks, and the capacity of the arc furnace has been increased as needed and many large three-phase AC arc furnaces of the capacity above 70 tons are operated. Accordingly, the unit power capacity tends to become larger, 500 KVA/ton or greater. As a result, the power trouble caused by the arc furnace, so-called flicker trouble on supply line, is becoming a major issue. More specifically, in an arc furnace which melts melting stocks, an arc generally occurs between the graphite electrode and melting stocks, but as the melting proceeds melting stocks fall and dislocate due to melt down around the electrode, the nearest melting stock position and length change, and so short arc or off arcing often occurs. This greatly varies the voltage applied to the arc furnace and arc current. In an AC arc furnace, a current flowing through the electrode becomes zero every half cycle, and so off arcing, and arc shorting frequently occurs. Then the arc is very unstable as viewed from the power system and becomes a significantly fluctuatable load. Such a load fluctuation changes the voltage of the power system and also varies the voltage in civil user's houses connected to the same power system, which may cause flickering of light.
Researches have been conducted from various angles to find out what causes the flickering in an arc furnace, and it was reported that the fluctuating level of reactive power due to a fluctuation of arc is an important factor to determine the degree of the flicker trouble ("Recent improvement in the stealmaking arc furnaces and their power supply", Technical Report (Part II). No. 72. (1978. Dec. 16), The Institute of Electrical Engineers of Japan).
With the above situations and the results of the researches considered, recent large AC arc furnace facilities are often provided with a flicker compensating apparatus as shown in FIG. 1. An AC arc furnace 3 is connected via a step-down transformer 2 for the furnace to an AC power system 1. The AC power from the power system 1 is stepped-down to a predetermined voltage by the transformer 2, then supplied to a movable electrode 4 inserted in the AC arc furnace 3 to generate an arc between the distal end of the electrode 4 and melting stocks placed in the furnace 3. The movable electrode 4 can be moved in the vertical direction so that the distance between its distal end and the melting stocks can be changed. The generation of the arc produces reactive power with a delayed phase. The continuous variation of the reactive power causes a flicker. In order to prevent an occurrence of the flicker and to compensate for the variation of the reactive power caused by the variation of the arc, a flicker compensating apparatus is provided in the power system 1 of the AC arc furnace 3.
A main portion of the flicker compensating apparatus comprises a capacitor 11 operating as high harmonic filter and improving the power factor to perform such pre-compensation as to cause the power system 1 to have a predetermined advanced phase in order to cancel the phase delayed by the reactive power, and a reactive load 17 for making a delay angle of the reactive power detected by a reactive power detector 12 constant. Each phase is provided with flicker compensator 18 formed of the capacitor 11 and a series circuit of the step-down transformer 15, thyristor 16, and reactive load 17.
A control section for the reactive load 17 comprises the reactive power detector 12 for detecting reactive power generated in the power system 1 based on the current and voltage thereof, a phase angle controller 13 for outputting a phase control angle instruction signal corresponding to the reactive power detected, a pulse generator 14 for outputting a phase control angle signal (pulse signal) according to this instruction signal, and the series circuit of the step-down transformer 15 and the reactive load 17 connected thereto via the thyristor 16 whose phase control angle is controlled by the pulse signal from the pulse generator 14, the series circuit connected to the power system 1. Controlling the phase angle of the thyristor 16 can control the current flowing through the reactive load 17. Consequently, such reactive power as to permit the power system 1 to always have a constant level of reactive power can be applied via the thyristor 16 to the power system 1 from the reactive load 17 by controlling the phase angle of the thyristor 16 based on a change in reactive power of the power system 1 detected by the detector 12.
The reactive power produced by the AC arc furnace 3 is irregular, making it difficult for the phase angle controller 13 to accurately predict it and properly control the phase control angle of the thyristor 16. Accordingly, a control delay of about 5 msec (quarter cycle) occurs even though the reactive power from the power system 1 is detected and controlled, resulting in insufficient compensation and undesirable contribution to causing the flickering in some cases. There is therefore a limit to the compensation effect of the flicker compensating apparatus for use in the AC arc furnace 3. In addition, the flicker compensating apparatus does not contribute so much to production of steel and, what is more, requires a great amount of facility expense.
Recent advancing power electronics technology, however, enhances the reliability of an AC/DC converter which converts AC power to DC power, and at the same time has led to development of large DC arc furnace employing the AC/DC converter. The DC arc furnace, as compared with the AC arc furnace 3, has proved to significantly contribute to reduction of the amount of consumption of the electric power, electrode and refractories, and reduction of the flickering. This effect of reducing the flicker occurrence is possible through the constant control of the arc current (DC current at the time of operation) by the AC/DC converter.
As should be apparent from the general P (effective power) vs. Q (reactive power) characteristic shown in FIG. 2, the level of the change of reactive power of a DC arc furnace, .DELTA.Qdc (=difference between reactive power Qs.sub.dc at the time of short-circuiting and reactive power Qo at the normal operation time), is approximately half the level of the change of reactive power of an AC arc furnace, .DELTA.Qac. Referring to the same diagram, DC indicates the P-Q characteristic curve of the DC arc furnace, and AC the P-Q characteristic curve of the AC arc furnace.
The reactive power Q of the DC arc furnace can be expressed by the following simple equation: EQU Q.apprxeq.Eo.multidot.Id.multidot.sin.alpha. (1)
where Eo is a no-load DC voltage, Id is an arc current and .alpha. is a phase control angle of a thyristor included in an AC/DC converter. Of these parameters, the no-load DC voltage Eo is specifically determined by the secondary voltage of the furnace transformer, and the arc current Id is constant due to a constant current control executed in the DC arc furnace at the time of operation and its sudden change is suppressed by the smoothing effect of the DC reactor. Therefore, an instantaneous change in reactive power is determined by the arc current Id and phase control angle .alpha., the latter being a large factor.
The reactive power in the AC arc furnace 3 is determined by 3.multidot.Ia.sup.2 .multidot.Xf where Ia is an arc current and Xf is the circuit reactance of the furnace.
As should be apparent from the above, the DC arc furnace and AC arc furnace have different mechanisms of generating reactive power. The level of a change in reactive power in the DC arc furnace can be reduced to a half that in the AC arc furnace due to the arc current being constant in the former furnace. However, there still is a case where the level of the flicker occurrent exceeds the limit, so that the DC arc furnace also needs a flicker compensating apparatus.
Conventionally, however, the flicker compensating apparatus for the AC arc furnace as shown in FIG. 1 is used for the DC arc furnace, and no specially designed flicker compensating apparatus for the DC arc furnace has existed. Naturally, there is about 5 msec of delay occurring in detection and control of the reactive power in the conventional flicker compensating apparatus for the DC arc furnace as in that of the AC arc furnace, and it is particularly difficult to properly cope with the instantaneous change in reactive power, resulting in insufficient compensating performance.